It has been known for many years that the use of one or more grounded electrodes near one or more corona wires in a gridded corona wire charger enhances DC corona emission current and also DC charging current (see, for example, R. M. Schaffert, Electrophotography, Focal Press, Revised and Enlarged Edition, 1975, p.234 (FIG. 92)). Until recently, however, little information has been published concerning the effects of grounded electrodes on the uniformity of corona charging current along the length of a corona wire, especially for AC charging using a control grid (AC scorotron).
As an AC charger ages during usage in a machine, conductive contamination compounds (corona chemistry by-products) can become deposited and build up on the surfaces of interior walls of a charger shield. The shield, inclusive of the endwalls, typically comprises an insulating plastic, such as used in primary chargers for Eastman Kodak's Ektaprint 2100, Ektaprint 3100, and Ektaprint IS110 electrophotographic machines. If a standard AC charger of this type is modified to have a grounded conductive floor surface, a problem can arise when conductive contamination compounds become deposited on the surfaces of the insulating endwalls and also deposited on the surface of ceramic insulation of a high voltage connector passing through an endwall stanchion to a terminal to provide high voltage from a power supply to a corona wire connected to the terminal. As a result of a high electric field between the terminal and a bare, grounded, conductive floor electrode, flashover electrical breakdown can occur, the flashover discharge passing between terminal and electrode across a contaminated endwall surface and thence over a contaminated surface of a ceramic insulator stanchion. The electrical breakdown over the surface of the insulation can produce an arc that can cause hard shutdown of an electrophotographic machine.
In DelVecchio, U.S. Pat. No. 3,978,379, it is stated in col. 2, lines 47-54, that "one way of increasing the plate current (i.e., charging current) and decreasing the shield current is to construct the shield so that the interior thereof opposite the photoreceptor has a dielectric surface that will increase the plate current component and decrease the shield current component by directing some of the upwardly directed (i.e., towards the shield) corona emissions downwardly toward the photoreceptor." The charger is presumably a DC charger. There is no disclosure in DelVecchio relating to charging current uniformity.
Nishikawa et. al., U.S. Pat. No. 4,053,769, describe a DC corona wire charger wherein a shield electrode is provided at its inner walls with an insulated layer. Nishikawa et al. notes that the insulating layer has a leak resistance which is inherent to the general property of the insulating material per se. This leak resistance is rapidly decreased to a small value as the voltage applied to the corona charge wire from the high voltage source is increased. As a result, if the flow of ions from the corona discharge wire arrives at the surface of the insulating layer, a constant surface electric potential is applied to the surface of the insulating layer, the constant surface potential being determined by the thickness and material of the insulating layer. This surface electric potential is stabilized at a value at which the leak resistance inherent to the property of the insulating material and the corona current are balanced with each other. The above description of Nishikawa et al. is thus consistent with a DC corona wire charger.
Fantuzzo, U.S. Pat. No. 4,754,305, discloses an AC charger using a corona wire coated with an electrically insulating layer, an electrically conducting U-shaped shield, and no grid.
Walgrove, U.S. Pat. No. 4,775,915, describes an AC corona wire charger comprising a non-conductive shell having no grid and further comprising an electrode inside the shell, the corona wire situated between electrode and a receiver. No dielectric coating on the electrode is disclosed.
Myochin et. al., U.S. Pat. No. 5,018,045, disclose a DC charger comprising a grounded conductive backplate physically separated from two DC-biased conductive sidewalls, all of which are physically separated from a DC-biased control grid which is parallel to the backplate, such that backplate, sidewalls and grid enclose a DC corona wire. FIG. 6 shows, as a comparative example, a similar prior art structure comprising grounded sidewalls such that the sidewalls and backplate are coated with electrically insulating Mylar.RTM. (polyethyleneterephthalate) plates.
In Benwood et. al., U.S. Pat. No. 5,642,254, some data are given for a 3-wire gridded AC charger comprising either a plastic floor, such as found for example in an Eastman Kodak Ektaprint 2100 electrophotographic machine, or a plastic floor covered by a bare metal grounded floor electrode. Different configurations are used in different examples. For a standard squarewave AC waveform (50% duty cycle), peak AC voltage .+-.8 KV, zero DC offset, and grid-to-collector spacing 0.060 inch, comparison of Tables 4 and 7 shows a 23% increase of the charging current using a grounded bare metal floor electrode instead of a standard plastic floor, as measured by a probe scanned along the length of the charger (parallel to the corona wires). On the other hand, a plastic floor used with extended plastic sideshields produced increases of charging current of 49% and 53%, using two different sets of corona wires (Tables 1, 3 and 4). Making both the floor and sidewalls conductive (without using extended sideshields) was also very beneficial, giving a 15% increase with peak AC voltage reduced by 1000 volts (to .+-.7 KV). The charging current uniformity, in a direction parallel to the corona wires, was apparently somewhat improved by the presence of the bare metal floor electrode, but the results are not conclusive on account of the use of different sets of corona wires in different examples.
Reddy and Litman, U.S. Pat. No. 5,568,230, describe a non-gridded AC charger comprising a dielectric-coated corona wire and a conductive shell further comprising an ozone neutralizing element in the form of a liner of the shell. The conductive shell may be grounded so that the charger acts as a neutralizer, or it may be biased with a DC potential so as to provide charging of the same polarity as the DC potential. The liner material described in this patent comprises an ozone-neutralizing layer, a support layer, and an adhesive layer. There is no disclosure regarding the electrical conductivities of these layers. There is some disclosure regarding the thickness as preferably in the form of a substantially thin coating at least 5 .mu.m thick.
It is well known that uniformity of charging is closely related to the uniformity of corona current emitted along the length of the corona wires. Corona current emitted from a wire typically shows significant fluctuations from one site to another on the wire. One can measure the resulting nonuniformity of the charging current along the length of a set of parallel corona wires in a gridded charger by means of a scanning probe, consisting of a thin grounded collector electrode inserted in a narrow slit cut in a large grounded plate electrode (slit perpendicular to wires). This is appropriately done by setting the collector-to-grid spacing at the same value as used for charging a photoconductor. It can be shown from a theoretical analysis that a deviation of output voltage on a charged photoconductor, measured in a direction perpendicular to the process direction as the photoconductor exits a charging station, is approximately proportional to the standard deviation of the scanned current divided by the mean scanned current. Hence, the use of a scanning probe to measure the fluctuations of charging current transmitted by the grid is a very useful predictor of the output uniformity performance of a gridded charger.
There is a general need to improve image quality in toned electrophotographic prints by reducing mottle and granularity related to primary charging nonuniformities, especially in solid areas having low to medium densities. This need is particularly acute for high quality color electrophotography. As is well known, prior art negative AC primary chargers produce much more uniform voltages on photoconductors than negative DC chargers. A primary negative AC charger of the invention provides inexpensive means for further improvements in negative AC charging current uniformity, and as a result, it gives a correspondingly improved voltage uniformity on a photoconductor.
There is a further need to improve the efficiency of an AC charger, inasmuch as the impedance of an AC charger is usually considerably higher, typically by a factor of about two, than that of a DC charger. The present invention advantageously provides a reduced impedance and hence a correspondingly higher efficiency of primary charging of a photoconductor.
There is yet a further need to improve the rigidity of a prior art AC charger body, especially when comprising a plastic shield, because a high tensile force on corona wires can result in a plastic shield becoming slightly bowed after long use, thereby causing a slackening of wire tension and a large scale nonuniformity of the charging current owing to such slackness. All modes of the present invention can provide improved rigidity at low cost.